Power line voltage transients, that is, voltage spikes, caused by, for example, the switching of loads to or from a branch of a power distribution system, lightning and so forth, can damage electronic appliances or devices, such as computers, connected to the power line. Such transients, while typically of short duration, may subject components in the devices to voltages and currents above their maximum operating limits. These transient voltages and currents may cause components to malfunction or explode, prematurely age, or may destroy them altogether. To prevent the transient voltages and associated currents from reaching the components, transient suppression circuits, sometimes referred to as surge suppression circuits, are connected to the device at the input from the power line, that is, at a point between the protected components and the power line.
A household/office, or single-phase power line normally includes three conductors, or wires, referred to as "line" or "phase", "neutral" and "ground." Transients may occur between the line and neutral wires, the line and ground wires or the neutral and ground wires, and thus, transient suppression circuits which can handle all three types of transients are desirable. Transient suppression circuits typically short-circuit the transients between the line and neutral wires. Such transients are commonly referred to as normal mode transients. The circuits typically direct to ground the transients between the line and ground wires or the transients between neutral and ground wires. These transients are referred to collectively as common mode transients. Three-phase power lines, for devices such as industrial equipment which draw higher currents, have 5 wires, three wires which correspond to the line, or phase, wires and a neutral wire and a ground wire. Transient suppression circuits for these power lines operate in basically the same manner as the three wire circuits and direct transients between the various wires to ground, if they employ common-mode protection.
In either case the suppression circuits include in the current directing paths energy-dissipating components, such as current limiting resistors or metal-oxide-varistors (MOV's). During a transient, the transient current is directed through one or more of these components, which essentially dissipate some of the associated energy. The remaining transient energy, and thus the current, is brought within a safe operating range.
U.S. Pat. No. 4,739,436 to Stefani et al., describes a voltage suppression circuit which uses MOV's connected between the various power line wires to direct transients to either the neutral wire or the ground wire, as appropriate. When power line voltages remain within a normal operating range the MOV's have a high impedance, and thus, they conduct essentially only leakage current. Accordingly, the power line current basically flows directly to the protected device. When a voltage transient between two power line wires occurs the voltage applied to the MOV's associated with the wires rises, which causes the MOV's to effectively lower their resistance. The transient current then flows through this low impedance path and is thus shunted away from the device components.
Known transient suppression circuits divert the currents associated with all voltage transients which are above a particular voltage level. The particular voltage level is the level that causes the MOV's to lower their impedances below those of the protected device components. The MOV's typically achieve such a lowered impedance at 320 volts AC RMS, or higher. However, this voltage level is below the levels required for certain high-voltage electrical safety tests required by, for example, United Laboratories (UL), for finished goods or appliances. Accordingly, the transient protection circuits with common-mode transient protection may interfere with the tests, if they are not removed or deactivated.
Specifically, each device must be tested by the manufacturer for, among other things, the effectiveness of its insulation. For example, the manufacturer must test the devices for conformance to UL 1950 (domestic) or IEC 950 (international) basic and supplementary insulation standards. These tests, called "dielectric withstand tests" and commonly referred to as the "hipot" tests, involve the shorting together of the line and neutral wires and the application of a relatively high voltage of approximately 1500 volts AC RMS between the ground and line wires. Components, such as filtering capacitors, connected between these wires may then conduct. If the wires are not properly insulated, or if the capacitors conduct a relatively large current, the device may be a safety hazard. A particular device which conducts current above predetermined, device-specific reactive and leakage current levels fail the test.
In known systems, the manufacturers either remove the power line transient suppression circuit from each device to be tested or de-activate the circuit before performing the hipot test. Otherwise, the suppression circuit provides a low resistance path for the current between the line and ground wires, and the device will necessarily appear to fail the test.
When the test is completed, the manufacturer must then re-insert or re-activate the suppression circuit. Either operation adds significant cost to the manufacturing process, which directly affects the product cost. What is needed is a voltage suppression circuit which can withstand the high voltages of the hipot test without conducting and still operate to suppress potentially harmful common mode voltage transients. Such a circuit may be in addition to or include circuitry which handles normal mode transients.